Selective catalytic reduction (SCR) systems have been used to reduce automotive emissions. Such systems typically add a gaseous or liquid reductant, such as ammonia or urea, to the exhaust gas stream from an engine to be absorbed onto a catalyst where the reductant reacts with nitrogen oxides in the exhaust gas to form water vapor and nitrogen. Various systems have been developed to add the reductant to the exhaust upstream of the SCR. Many systems use multiple pressure sensors located in the emissions control system to meter the amount of reductant added to the exhaust stream.
One approach to measure the amount of reductant added to the exhaust stream is described in U.S. Pat. No. 6,167,698. Reductant is supplied to the exhaust stream through an exhaust gas purification system. The method determines the amount of reductant added to the exhaust stream based on the pressure differential between the exhaust stream and the reductant supply system.
The inventors herein have recognized that, for a variety of reasons, such as the caustic environment in the exhaust gas, purification system, one of more of the pressure sensors in the exhaust gas purification system may degrade.
One approach to address at least some of the above issues includes a method for controlling operation of a reductant injection system having a pump, injector, and pressure sensor located upstream of the injector in the reductant injection system. The method may comprise, under engine starting conditions: disabling the pump and opening the injector; and indicating degradation of the sensor when the sensor output, during the disabled pump operation and opened injector, disagrees with exhaust pressure. Specifically, under such conditions, the pressure sensor should agree with exhaust pressure, and disagreement may indicate that the sensor reading is erroneous.
Alternatively, or in addition, the method may include, adjusting actuation of the pump and/or injector during engine operation and reductant injection to compensate for any disagreement between the sensor and exhaust pressure, thereby providing improved reductant injection control. For example, sensor drift can be identified and corrected. However, if the sensor error is greater than a threshold, the system may transition to adjusting the pump and/or reductant injector actuation independent of the pressure sensor.